Passive components today exist as a carrier substrate with built-in passive compositions usually derived from resistor or dielectric thick film technology, which are terminated by metal conductors. The components are mounted on a surface of a printed wiring board (PWB) one at a time with pick and place equipment, and connected electrically to a circuit by one of several complex processes which may involve adhesives, fluxes, solder composition, wave soldering, or reflow.
As the demand for miniaturizing electronic devices increases, both circuit density and the density of components per unit area have dramatically increased. The number of components has risen exponentially, necessitating the shrinking of component size. Since the design of smaller, denser boards is approaching the practical limit for today's technology, circuit designers have maximized real-estate and cannot add components unless they become much smaller for surface mounting, or buried within the inner layers, i.e., vertical stacking of components. It is desirable to have components resulting from thick film compositions within the embedded layers. Thick film resistor and dielectric compositions are an existing technology, such that a viscous thick film composition is screen-printed in a desired design, then fired at a temperature to burn out organic components and sinter the inorganics. The result is a thick film component embedded into the circuitry.
Although thick film resistors are thin enough to embed, they cannot be printed directly on a printed wiring board because of a firing step in the manufacturing process. The resistor composition can be printed and fired on a fireable substrate, and then laminated to the board. However, there is a tendency for the fired components to crack during lamination, thereby reducing yields. A method is needed to allow lamination of thick film components to a printed wiring board with reduced incidence of cracking.
A further obstacle to using thick film resistors on a printed wiring board is that they cannot be laser-trimmed using present technology. Laser trimming is a method of adjusting the resistance of a fired thick film resistor. The resistance is measured, and the change in width needed to achieve the proper resistance is calculated. A laser makes a cut through the thick film resistor, partway across the current path. This cut reduces the effective width of the thick film resistor and raises the resistance to the desired value. When trimming is done on a printed wiring board, the laser cuts through the thick film resistor and burns the board. The burned material can form a conductive carbon bridge across the cut path, which can lower the resistance of the thick film resistor and/or cause drifting of the resistance. The present invention solves the problem of laser trimming printed thick film resistors on organic substrates such as printed wiring boards and, also, solves the problem of cracking during lamination.